Cheese is a preserved dairy product where microorganisms are fundamental ingredients, not contaminants. The transformation of liquid milk into a solid, flavorful, and shelf-stable food depends entirely on the controlled activity of bacteria, molds, and yeasts. This biological framework establishes the initial texture and flavor foundation, setting the stage for complex changes during aging.
The Essential Starter Cultures: Primary Fermentation
The initial step relies on primary fermentation carried out by Lactic Acid Bacteria (LAB), intentionally introduced as starter cultures. These bacteria, belonging to genera like Lactococcus and Streptococcus, consume lactose and convert it into lactic acid. This conversion rapidly lowers the milk’s pH, which is the first step in coagulation, or curdling.
The acidification process causes the main milk protein, casein, to destabilize and form a solid gel, separating the curds from the liquid whey. This drop in pH also creates an inhospitable environment, preventing the growth of undesirable organisms and acting as a natural preservation mechanism. Cheesemakers select cultures based on their optimal operating temperature: mesophilic cultures thrive at moderate temperatures (e.g., Cheddar or Gouda), while thermophilic cultures require higher heat (e.g., Parmesan or Mozzarella). This choice dictates the initial moisture, texture, and acidity profile of the fresh curd, influencing the final cheese type.
Secondary Microbes: Developing Flavor, Aroma, and Texture
Once the curd is formed, secondary microbes take over during ripening, generating the unique characteristics of aged cheeses. These organisms break down remaining fats and proteins through enzymatic activity, creating volatile flavor compounds. This secondary activity defines the final sensory profile, moving the cheese beyond a simple acidic curd.
In blue cheeses, the mold Penicillium roqueforti is inoculated directly into the curd, forming characteristic blue-green veins. This mold possesses strong lipolytic activity, breaking down milk fats into free fatty acids, which are metabolized into methyl ketones and secondary alcohols. These compounds are responsible for the sharp, piquant, and sometimes spicy flavor profile associated with cheeses like Roquefort and Gorgonzola.
Soft, surface-ripened cheeses, such as Camembert and Brie, rely on the growth of the mold Penicillium camemberti on the exterior. This mold forms a dense, white rind and releases powerful enzymes that cause proteolysis (protein breakdown), softening the cheese from the outside toward the center. This process creates the buttery, creamy texture and the subtle, earthy aromas that are hallmarks of these varieties.
Washed-rind cheeses, like Limburger or Époisses, owe their pungent aroma and reddish-orange surface color to the bacteria Brevibacterium linens. This bacteria is encouraged to grow by repeatedly washing the surface with brine or alcohol. B. linens produces sulfur-containing compounds responsible for the intensely savory smell, and its enzymes contribute to the complex flavor that penetrates the interior.
In hard cheeses like Swiss varieties, Propionibacterium freudenreichii metabolizes lactic acid into propionic acid, acetic acid, and carbon dioxide. The carbon dioxide gas creates the distinct holes, or “eyes,” in the cheese matrix.
Quality Control and Preventing Pathogens
The intentional use of microbes is balanced by safety protocols designed to prevent the growth of harmful bacteria. Pasteurization, a common thermal treatment, serves as the primary defense, eliminating vegetative pathogens such as Listeria monocytogenes, Salmonella species, and pathogenic E. coli strains before cheesemaking begins. This allows the introduced starter cultures to dominate the environment without competition from contaminants.
For cheeses made from unpasteurized (raw) milk, regulatory bodies in the United States require a minimum aging period of 60 days at a temperature no lower than 35°F (1.67°C). This regulation is based on historical data suggesting that acidity, salt content, and extended aging time are sufficient to inactivate most bacterial pathogens. However, modern studies show that some pathogens can survive the 60-day period in certain cheese types, leading to a re-evaluation of the rule.
Beyond initial processing, the industry employs the Hazard Analysis Critical Control Point (HACCP) system. HACCP involves identifying specific points in production where hazards can be controlled, such as monitoring milk storage temperature or ensuring the correct pH is reached after acidification. This systematic application of checks and balances provides a continuous barrier against contamination throughout the production and ripening cycle.